Phentolamine and Rat Aortic Smooth Muscle Responsiveness to Potassium Chloride, Isoproterenol and Norepinephrine

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Abstract
Rat aortic smooth muscle responsiveness to potassium chloride and isoproterenol in the presence of the α-adrenergic blocker phentolamine was investigated. A significant decrease in the response of the vascular smooth muscle of aorta to low concentrations of potassium chloride (KC1; 8–12 mM) in the presence of phentolamine was observed. However, no effect of phentolamine was seen at KC1 concentrations greater than 16 mM. Isoproterenol-induced relaxation was significantly increased in aortic smooth muscle incubated with phentolamine. Phentolamine also attenuated the contraction induced by high concentrations of isoproterenol. No long-lasting effect of the α-adrenergic blocker was apparent since the response to norepinephrine was not altered following a 1-hour washout of phentolamine after the isoproterenol dose-response curve. The results of this study suggest that: (i) phentolamine has no long-lasting effects on rat aortic smooth muscle; (ii) since the rat thoracic aorta has a sparse adrenergic innervation, the decrease in responsiveness to KC1 in the presence of phentolamine is most likely due to a nonspecific effect on the vascular smooth muscle as opposed to blocking the effects of norepinephrine released from adrenergic nerve terminals, and (iii) the increased relaxation response to isoproterenol may be due to a blockade of the α-receptors activated by isoproterenol.